It is a longstanding problem in the field of securing large, multi-component vehicle joints through the use of U-bolts. An example of such joints occurs in the attachment of wheel axles for large commercial trucks.
Referring to FIG. 1, an example of a multi-component U-bolt joint for a large truck is shown. A highly useful application of the adaptive inventive process is to secure a heavy-duty truck axle to the truck suspension leaf spring. The axle is shown inverted 180 degrees which is a common orientation to assemble these vehicle components. In the example, a U-bolt joint 20 including a pair of U-shaped bolts (referred hereinafter as U-bolt or U-bolts) 24 are used to secure truck axle 30 to a leaf spring stack 36 as generally shown. Each U-bolt 24 includes a long shaft or leg 40 defining an axis 50 having threaded portions 54 on the respective ends of the legs. Depending on the design of the truck, the joint 20 may further have a mounting plate 70 (shown integral with the axle in the example) and one or more spacers or risers 80 between the leaf springs and axle. Depending on the vehicle and/or joint design, the spacers 80 may be small/thin or six (6) or more inches in height.
A fastener nut 60 is threaded on each threaded portion 54 and tightened to apply a compressive force on the joint 20 to secure the axle 30 to the leaf spring 36. This is typically done at each wheel. Examples of U-bolts for heavy-duty truck axles mounts are ¾ or ⅞ inches in diameter and may range for 6 inches to more than 24 inches in length from crown 44 to leg end. Shims 48 may be used between the crown 44 and leaf spring stack 36 as generally shown. Common target torque values for each nut may range from around 300 foot-pounds (ft-lbs) to 450 foot-pounds or more depending on the size of the U-bolts and application. Applications other than on heavy trucks and having different U-joint subcomponents and sizes of U-bolts 24 known by those skilled in the art may be used. It is further contemplated that application of the inventive adaptive stabilization process 10 may be used on fastening bolts or studs other than U-bolts, and in different sizes, lengths and fastener configurations as described and illustrated herein as known by those skilled in the art.
These axle joints 20 are problematic in the vehicle assembly process due to the relatively non-precision nature of such large, heavy components often made from rough cast or forged steel or iron. Where precision is required, localized machining may be employed. However, due to the unprotected, exterior vehicular environment and heavy-duty use, even precision surfaces are often coated with thick rust and corrosion resistant protectants and exterior lubricants, such as heavy greases. The protective coatings and other imprecise features of each component prevent precision alignment or stack-up in any orthogonal direction. As the wheel axles support the wheels and the entire weight of the vehicle, it is important that these joints form stable, secure connections of the axle to the truck suspension.
Conventional techniques to torque the U-bolt nuts 60 to proper specifications have historically been problematic and inefficient both in initial assembly and during the initial stages of truck usage. In order to reduce the time to assemble large trucks and amount of manual labor, automated nut driving devices have been employed to simultaneously tighten the nuts 60. In the example shown in FIG. 3, a spindle device 82 includes four independently monitored and controlled nut drivers 94 as generally shown. Each driver includes a socket 96 suitable for engaging a nut, for example U-bolt nut 60 shown in FIG. 1.
Spindle devices 82 are rotatably connected to electric motors in electronic communication with programmable controllers 98 which raise and lower the drivers and precisely control the rotational movement of each nut driver 94. As schematically shown in FIG. 9, exemplary spindle device 82 is connected to a larger control unit 84 including, for example, one or more computer processors 86, controllers 88, short and/or long term memory storage devices 90, and communication hardware 91 in electrical and data communication with one another to store, send and receive instructions to operate spindle device 82 through communication link 92 as generally shown. Monitoring instruments and associated hardware and software (not shown) to operate and guide spindle devices 82 (only one shown) allow for real time measurement, recordation and processing of many different metrics, for example rotational torque, angular rotation, yield, amperage and time for each individual nut driver 94. These metrics and data can be stored, analyzed and visuals presented for use by technicians for applications such as process 10 as more fully described below. One example of an industrial nut driver is QST90-1000CT model number 8435 6090 10 manufactured by Atlas Copco. Information on the example can be found at www.atlascopco.com/Atlas Copco Industrial Power Tools Catalog the entire contents of the example is incorporated herein by reference.
Even sophisticated prior fastener and nut driving devices suffered disadvantages in securing the imprecise axle U-bolt joints 20. For example, even under synchronized and simultaneous turning of the four U-bolt nuts 60, the several joint components may shift relative to one another and skew the initial geometric stack thereby forcing the U-bolts 24 to skew or cock. On this occurrence, often one leg 40 of a U-bolt 24 may shift relative to the mounting plate leaving less threads for the nut to engage and lengthening the exposed threads on the other leg for the respective U-bolt nut to engage.
As the nut drivers 94 continue to rotate the nuts 60 and apply torque, the nut on the U-bolt leg with reduced exposed or available threads 54 extending above the mounting plate 70 reaches a predetermined torque value sooner than the other leg which keeps rotating that nut until the specified torque is reached. Once the torque value is reached a driver 94 will stop so as to not over-torque the nut and either strip the nut threads or facture the U-bolt leg 40. If one nut hits the predetermined torque value limit well prior to the other leg, the joint is further subject to geometric distortion and increased stresses on the U-bolt which can lead to bolt fracture and joint failure at assembly.
It is also common due to the long length of the U-bolt legs and relatively lower lateral strength of the U-bolts compared to the heavy-duty joint sub-components, that the U-bolts will laterally yield or permanently bend to accommodate shifting of the subcomponents on the high compressive forces imparted by torqueing the nuts. It has been discovered that when a U-bolt joint assembly becomes skewed or misaligned during assembly using conventional compression techniques, the nut torque or resistive force to further compression is at least in part generated by lateral bending or work hardening, elastic and/or plastic deformation, of the U-bolt legs and not from elastic (recoverable) axial elongation of the U-bolt legs 40 which is required of a mechanical fastener for proper residual torque on the nut and lasting compression of a stabilized joint.
An example of a torque versus degree of angular rotation data tracing of a conventional truck axle U-bolt joint tightening process is shown in FIG. 4. In the example, two U-bolts each including two shafts 40 and each shaft 40 requiring a nut 60 (four (4) legs and four (4) nuts total 102, 110, 114 and 120). At angle zero (0) tightening of all four nuts 102, 110, 114 and 120 begins simultaneously by a spindle driver similar to 82 toward a target torque 130 value (in the example, 275 ft-lbs). The exemplary test tracing shows a common phenomenon for tightening U-bolt joints. As the example shows, the first nut 102 reaches the target torque 130 at point 134 at about 350 degrees of rotation and the nut driver stops. The third nut achieves the target torque 130 in about 460 degrees of rotation at 138. The second nut 110 achieves the target torque after about 825 degrees at 140. The fourth nut 120 achieves that target torque 130 at over a 1000 degrees of rotation.
It is well documented that such U-bolt joints suffer from poor residual torque, i.e. a subsequently measured nut torque following an initial torqueing after a joint's residual stresses and temperature ease and the joint “relaxes.” Due to the above-described problems, there is also large variation in the residual torques from nut-to-nut and joint-to-joint.
In order to cure the deficiencies in the conventional initial torquing process, one or more checks of the nut torques must be manually made by technicians to ensure the nuts are to the specification torque values. If any of the torque values are too low, the nuts have to be manually re-torqued to specification values by a technician in the assembly plant or out in the field, for example prior to new truck delivery and before the truck is placed into service. Due at least in part to the difficulties in conventional initial assembly as described and nature of the joint, manufacturers recommend additional U-bolt nut torque checks at regular intervals of service. The manual checking and re-torquing is manually intensive and in difficult areas on the underside of the vehicle.
Therefore, there is a need to improve upon these assembly deficiencies and produce a more efficient assembly process forming more robust U-bolt joints requiring fewer subsequent manual processes.